Investigation of CO2 retention efficiency and biodiesel production potential in different growth conditions of chlorella vulgaris microalgae culture

In this study; it was aimed to determine the CO2 capture efficiency and biodiesel potential of Chlorella vulgaris (C. vulgaris) microalgae culture grown in a vertical photobioreactor. In the study, a photobioreactor made of plexiglass with 5 cm inner diameter and 100 cm height was used. CO2 capture efficiency of the culture was investigated by giving gas mixtures containing different proportions of CO2 (400 ppmv, 15% and 90% by volume) to the photobioreactor. The CO2 + N2 + O2 gas mixture, containing 400 ppmv CO2, represents the CO2 concentration in the air. The CO2 + N2 + O2 gas mixture, which contains 15% CO2, represents the flue gas composition resulting from coal-fired thermal power plants. The CO2 + O2 gas mixture, which contains 90% CO2, represents the flue gas composition resulting from coal-fired thermal power plants using Oxy-Fuel combustion technology. Other variable parameters are illumination duration (7 000, 11 000 and 15 000 lux) and illumination duration (12 hours, 16 hours and 24 hours of illumination). Samples are taken from the photobioreactors every 24 hours. Dry weight (g/L), optical density (684 nm and 750 nm), chlorophyll-a (665 nm), chlorophyll-b (645 nm), carotenoid (480 nm) and pH were measured for each sample. As a result of the analyzes, adaptation period (hours), reproductive efficiency (g (dry weight)/L/day), maximum specific growth rate, (µmax, 1/day) and CO2 capture efficiency were calculated for each test. Variable parameters that give the best results in terms of CO2 retention efficiency were determined and under this parameter conditions, microalgae culture was exposed to nitrogen stress and the lipit ratio in its structure was examined. The dry weight (max ~ 2 g/L) at 7000 lux was 2.5 and 3.8 times higher than those obtained at 11 000 lux and 15 000 lux, respectively. The highest biomass concentration (max ~ 2 g/L) was obtained in 24 hours of illumination. It was 1.8 and 1.5 times higher than 16 hours and 12 hours of illumination, respectively. Dry weight (max ~ 1.47 g/L) at 15% CO2 was 5 times higher than 400 ppmv. Reproductive efficiency and CO2 capture efficiency for the gas mixture containing 15% CO2 were determined as 0.14 g/L/day and 0.25 g/L/day, respectively. Microalgal growth was inhibited at 90% CO2. While the lipid content was 10% at the beginning for the test at 15% CO2, it was decreased to 3% at the end of the test. The growth was inhibited at 15% CO2 concentration under nitrogen stress in the photobioreactor. Therefore, nitrogen stress tests were conducted in an erlenmeyer flask at 400 ppmv CO2. It was determined that there was a better growth under 75% nitrogen stress (1.3 times higher) compared to 100% nitrogen stress. Moreover, RuBisCO for 75% nitrogen stress was 2.5 times higher than %100 nitrogen stress. However, In the 100% nitrogen stress test, lipid content was 1.2 times higher compared to the 75% nitrogen stress test. KEYWORDS: CO2 Capture, Microalgae, Chlorella vulgaris, Flue Gas, Thermal Power Plants, Photobioreactor, Nitrogen Stress, Lipid, RuBisCO, Biodiesel.